The present invention relates in general to a system for time compression or expansion of electrical signals, particularly video signals, in which samples of the signals are interpolated and written in a memory at a first clock rate and then read out from the memory at a second clock rate.
In various fields of communications technology, particularly in the television technology, it is frequently desired that processed signals be spread out in time or shortened in time. As known, the signals to be processed in such a manner are first written into a memeory at a first clock frequency and thereafter read out from the memory at a second clock frequency deviating from the first one. Inasmuch as the employed memory is a digital one, it is first necessary to convert the processed analog signals into corresponding digital signals and after their compression or expansion, if desired, the signals are reconverted into analog form. One of the most important applications of time expansion or compression in the field of television technology is the so-called electrical zooming, in which, apart from linear changes of processed magnitudes, also changes in vertical or horizontal direction only, and various distortions of the TV picture, are also possible. Another use of the expansion or compression system is for format modification, for example for the reproduction of cinemascope-type film, standard converters for converting TV signals of numbers of lines and/or picture frequencies.
A system of this kind is known for example from RCA Review, Vol. 42, No. 1, March, 1981, pp. 3-59, in which the compression is achieved by deleting some of the scanned values of a video signal. This method, however, in the case of predetermined sampling frequencies produces interferences in the reproduced image. In another proposal, the above article (page 43 et seq.) describes sampling of the signal to be processed at a frequency which differs from the original sampling frequency. To this end it is necessary to determine the resulting new samples by interpolation. If these new samples are written into a memory and subsequently read out from the memory at the original frequency, then a compression and/or expansion is achieved. The disadvantage of this prior-art method is the necessity to use an additional sampling frequency which differs from that at which the signals were originally sampled. Particularly when using a variable compression and/or expansion factor difficulties will result during the synchronization of individual component parts of the interpolation circuit and of the memory.